1. Field of the Invention
The present invention relates to a radio system, and more particularly to a transmission power control method and apparatus for mobile communications using a CDMA (Code Division Multiple Access) system.
2. Description of Related Art
As is well known, since a plurality of users share the same frequency band in a CDMA system, signals from other users interfere with the signal of a user, thereby degrading the communication quality of the user. In addition, when a first mobile station near a base station and a second mobile station far away from the base station perform communications at the same time, a transmitted signal from the first mobile station will be received at higher power by the base station, and a transmitted signal from the second mobile station will be received at lower power.
Thus, a near-far problem arises in that channel quality will be greatly degraded in communications between a base station and far away mobile stations owing to the interference from mobile stations near the base station. In view of this, transmission power control has been studied as a technique for overcoming the near-far problem. The transmission power control in a transmitting station is carried out by controlling transmission power in such a manner that the received power by a receiving station, or the SIR (Signal-to-Interference power Ratio) obtained from the received power is kept constant, independent of the locations of the transmitting station, thereby providing uniform channel quality throughout the service area. In particular, in reverse (from mobile station to base station) channels, the transmission power control in respective mobile stations is performed such that the received power of signals transmitted from the mobile stations and received by the base station, or the SIR associated with the received power is kept constant at the base station.
Since CDMA systems consider the interferences from the other users as white noise, equivalent noise power increases with the number of users, and the capacity in terms of the number of subscribers in a cell is determined on the basis of the received SIR which can provide predetermined channel quality.
On the other hand, the received SIR is kept constant in forward (from base station to mobile station) channels. This is because an intended channel signal travels from a base station to a mobile station through the same propagation paths as the signals for other users which cause the interference, and undergoes the same fluctuations as the interference waves, which fluctuations include long-term, short-term, and instantaneous fluctuations. Therefore, the received SIR is constant in forward channels, and hence, the transmission power control is not required when taking account of only the interference in the same cell.
The CDMA system, which handles the interference as white noise, however, must take account of the interferences from other cells because it shares the same frequency band with the adjacent cells. Although the interference power from other cells is caused by instantaneous fluctuations due to Rayleigh fading as the interference power in the same cell, the fluctuations differ from those of the desired signal.
According to the CDMA system standard by TIA (Telecommunications Industry Association), the forward transmission power control is not performed basically, but the transmission power of the base station to a mobile station is increased when a frame error rate at the base station exceeds a predetermined threshold level. This is because a large quantity of transmission power changes will increase the interference to other cells. This conventional system, however, cannot follow the instantaneous fluctuations due to the interferences from other cells, which are caused by the transmitted signals from the base stations of other cells.
A transmission power control method based on a closed loop control using a transmission power control bit is known as a conventional transmission power control method that can track the instantaneous fluctuations.
FIGS. 1A and 1B show an example of the transmission power control method based on the closed loop control when the base station communicates with a mobile station within the cell. As shown in FIGS. 1A and 1B, the mobile station despreads a received signal from the base station at step S102, and then, measures the received power of the desired signal from the base station and that of interference at step S122. Subsequently, the mobile station calculates a received SIR based on the measured results at step S124, compares the received SIR with a reference SIR at step S126, and determines the transmission power control bit for controlling the transmission power of the base station on the basis of the comparison result at step S128. Then, the mobile station forms a frame which includes the transmission power control bit at step S130, and transmits it to the base station. In parallel with this, the mobile station demodulates a signal transmitted from the base station at step S132, extracts a transmission power control bit at step S134, and decides its own transmission power in accordance with the command of the transmission power control bit sent from the base station at step S136.
Similarly, the base station despreads a received signal from the mobile station at step S142, demodulates it at step S172, extracts a transmission power control bit at step S174, and determines its own transmission power in accordance with the command of the transmission power control bit sent from the mobile station at seep S176. Furthermore, the base station measures the received power of the desired signal from the mobile station and that of interference at step S162, calculates received SIR at step S164, compares the received SIR with a reference SIR at step S166, and determines a transmission power control bit for controlling the transmission power of the mobile station on the basis of the measured result at step S168. The base station then forms a frame by inserting the transmission power control bit into a signal to be transmitted at step S170, and sends it to the mobile station.
In the conventional closed loop transmission to power control method described above in connection with FIGS. 1A and 1B, high reliability of the transmission power control bit is required. This requirement arises out of the possibility that an erroneous transmission power control bit due to degradation of the communication path might result in power control opposite to the required control. This will increase interference to other users because of excessive transmission power, or result in insufficient channel quality because of the lack of transmission power.
Error correction coding for the transmission power control bit is effective in improving the reliability of the transmission power control bit. In particular, convolutional coding/Viterbi decoding, which has high correcting ability for random errors, is effective for cellular CDMA systems because errors are made random by the bandwidth spreading in the CDMA system. The Viterbi decoding, however, requires a path memory whose path history length is five to six times the constraint length of a convolutional code. Therefore, decoding of the transmission power control bit causes a delay corresponding to the path history length of the path memory. This presents a problem in that instantaneous changes in communication path cannot be followed, and this hinders high accuracy transmission power control.